Three-dimensional phase-change memory array

ABSTRACT

A three-dimensional phase-change memory array. In one embodiment of the invention, the memory array includes a first plurality of diodes, a second plurality of diodes disposed above the first plurality of diodes, a first plurality phase-change memory elements disposed above the first and second plurality of diodes and a second plurality of memory elements disposed above the first plurality of memory elements.

RELATED APPLICATION INFORMATION

This application is a divisional of U.S. patent application Ser. No.11/522,584, entitled “Three-Dimensional Phase-Change Memory” and filedon Sep. 18, 2006, now U.S. Pat. No. 7,391,045, the disclosure of whichis incorporated by reference in its entirety herein.

FIELD OF THE INVENTION

The present invention is related to electrically operated phase-changememory. In particular, the present invention relates to athree-dimensional memory array comprising electrically operatedphase-change memory.

BACKGROUND OF THE INVENTION

Programmable resistance memory elements formed from materials that canbe programmed to exhibit at least a high or low stable ohmic state areknown in the art. Such programmable resistance elements may beprogrammed to a high resistance state to store, for example, a logic ONEdata bit or programmed to a low resistance state to store a logic ZEROdata bit.

One type of material that can be used as the memory material forprogrammable resistance elements is phase-change material. Phase-changematerials may be programmed between a first structural state where thematerial is generally more amorphous (less ordered) and a secondstructural state where the material is generally more crystalline (moreordered). The term “amorphous”, as used herein, refers to a conditionwhich is relatively structurally less ordered or more disordered than asingle crystal and has a detectable characteristic, such as highelectrical resistivity. The term “crystalline”, as used herein, refersto a condition which is relatively structurally more ordered thanamorphous and has lower electrical resistivity than the amorphous state.

The phase-change materials may be programmed between differentdetectable states of local order across the entire spectrum betweencompletely amorphous and completely crystalline states. That is, theprogramming of such materials is not required to take place betweencompletely amorphous and completely crystalline states but rather thematerial can be programmed in incremental steps reflecting (1) changesof local order, or (2) changes in volume of two or more materials havingdifferent local order so as to provide a “gray scale” represented by amultiplicity of conditions of local order spanning the spectrum betweenthe completely amorphous and the completely crystalline states. Forexample, phase-change materials may be programmed between 3 or moreresistive states to store more than 1 bit of information in one memorycell. For example, phase-change materials may be programmed between fourresistance states to store 2 bits of information in a memory cell.

A volume of phase-change material may be programmed between a moreordered, low resistance state and a less ordered, high resistance state.A volume of phase-change is capable of being transformed from a highresistance state to a low resistance state in response to the input of asingle pulse of energy referred to as a “set pulse”. The set pulse issufficient to transform a volume of memory material from the highresistance state to the low resistance state. It is believed thatapplication of a set pulse to a volume of memory material changes thelocal order of at least a portion of the volume of memory material.Specifically, it is believed that the set pulse is sufficient to changeat least a portion of a volume of memory material from a less-orderedamorphous state to a more-ordered crystalline state.

A volume of memory material is also capable of being transformed fromthe low resistance state to the high resistance state in response to theinput of a single pulse of energy which is referred to as a “resetpulse”. The reset pulse is sufficient to transform a volume of memorymaterial from the low resistance state to the high resistance state.While not wishing to be bound by theory, it is believed that applicationof a reset pulse to a volume of memory material changes the local orderof at least a portion of the volume of memory material. Specifically, itis believed that the reset pulse is sufficient to change at least aportion of the volume of memory material from a more-ordered crystallinestate to a less-ordered amorphous state.

The use of phase-change materials for electronic memory applications isknown in the art. Phase-change materials and electrically programmablememory elements formed from such materials are disclosed, for example,in U.S. Pat. Nos. 5,166,758, 5,296,716, 5,414,271, 5,359,205, 5,341,328,5,536,947, 5,534,712, 5,687,112, and 5,825,046 the disclosures of whichare all incorporated by reference herein. Still another example of aphase-change memory element is provided in U.S. patent application Ser.No. 09/276,273, the disclosure of which is also incorporated herein byreference.

It is important to be able to accurately read the resistance states ofprogrammable resistance elements which are arranged in a memory array.The present invention describes an apparatus and method for accuratelydetermining the resistance states of programmable resistance elementsarranged as memory cells in a memory array. Background art circuitry isprovided in U.S. Pat. No. 4,272,833 which describes a reading apparatusbased upon the variation in the threshold levels of memory elements, andU.S. Pat. No. 5,883,827 which describes an apparatus using a fixedresistance element to generate reference signals. Both U.S. Pat. No.4,272,833 and U.S. Pat. No. 5,883,827 are incorporated by referenceherein.

SUMMARY OF THE INVENTION

An aspect of the present invention is a memory array, comprising: aplurality of first isolation elements; a plurality of second isolationelements disposed above the first isolation elements; a plurality offirst phase-change memory elements disposed above the second isolationelements, each of the first memory elements electrically coupled to acorresponding one of the first isolation elements; and a plurality ofsecond phase-change memory elements disposed above the first memoryelements, each of the second memory elements electrically coupled to acorresponding one of the plurality of second isolation elements.

Another aspect of the present invention is a memory array, comprising: aplurality of lower first conductive lines; a plurality of upper firstconductive lines disposed above the lower first conductive lines; aplurality of lower second conductive lines disposed above the upperfirst conductive lines, the lower second conductive lines crossing thelower and upper first conductive lines; a plurality of upper secondconductive lines disposed above the lower second conductive lines, theupper second conductive lines crossing the lower and upper firstconductive lines; a plurality of first phase-change memory cells, eachof the first phase-change memory cells coupled between a correspondinglower first conductive line and a lower second conductive line; and aplurality of second phase-change memory cells, each of the secondphase-change memory cells coupled between a corresponding upper firstconductive line and a corresponding upper second conductive line.

Another aspect of the present invention is an integrated circuit,comprising: a memory array, comprising: a plurality of first addresslines, each of the first address lines having a width W₁; a plurality ofsecond address lines crossing the first address lines, each of thesecond address lines having a width W₂; and a plurality of phase-changememory cells, each of the memory cells electrically coupled between acorresponding one of the first address lines and a corresponding one ofthe second address lines, wherein the cell size of the memory array isless than 4(W₁) (W₂). It is, of course, noted that the notation 4(W₁)(W₂) means: 4 times W₁ times W₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 18 are isometric views illustrating the fabrication ofan embodiment of a three-dimensional memory array of the presentinvention;

FIG. 19A is cross-sectional view through XA-XA of the three-dimensionalmemory array shown in FIG. 18;

FIG. 19B is a cross-sectional view through XB-XB of thethree-dimensional memory array shown in FIG. 18;

FIG. 19C is a cross-sectional view through YA-YA of thethree-dimensional memory array shown in FIG. 18;

FIG. 19D is a cross-sectional view through YB-YB of thethree-dimensional memory array shown in FIG. 18;

FIG. 19A′ is cross-sectional view through XA-XA with overlap of theupper and lower diode strips and overlap of the corresponding upper andlower row lines;

FIG. 19B′ is a cross-sectional view through XB-XB with overlap of theupper and lower diode strips and overlap of the corresponding upper andlower row lines;

FIG. 19C′ is a cross-sectional view through YA-YA with overlap of theupper and lower memory strips and overlap of the corresponding upper andlower column lines;

FIG. 19D′ is a cross-sectional view through YB-YB with overlap of theupper and lower memory strips and overlap of the corresponding upper andlower column lines;

FIG. 20 is an isometric view of the three-dimensional memory array shownin FIG. 18 with the dielectric layers removed;

FIG. 21A is a cross-sectional view of the three-dimensional memory arrayshown in FIG. 20 through the cross-section XA-XA;

FIG. 21B is a cross-sectional view of the three-dimensional memory arrayshown in FIG. 20 through the cross-section XB-XB;

FIG. 21C is a cross-sectional view of the three-dimensional memory arrayshown in FIG. 20 through the cross-section XC-XC;

FIG. 21D is a cross-sectional view of the three-dimensional memory arrayshown in FIG. 20 through the cross-section XD-XD;

FIG. 22 shows a schematic diagram of the three-dimensional memory arrayshown in FIG. 20;

FIG. 23A is a top view of an embodiment of a three-dimensional memoryarray of the present invention where there is no overlap between the rowlines and no overlap between the column lines;

FIG. 23B is a top view of an embodiment of a three-dimensional memoryarray of the present invention where there is overlap between the rowlines but no overlap between the column lines;

FIG. 23C is a top view of an embodiment of a three-dimensional memoryarray of the present invention where there is no overlap between the rowlines but overlap between the column lines;

FIG. 23D is a top view of an embodiment of a three-dimensional memoryarray of the present invention where there is overlap between the rowlines and overlap between the column lines;

FIG. 24 is a cross-sectional view of a memory cell of the presentinvention showing an alternate electrode structure;

FIG. 25A is a top view of a three-dimensional memory array from FIG. 23Ashowing a lateral cross section of a unit volume; and

FIG. 25B is a top view of a two-dimensional memory array showing alateral cross section of a unit volume.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 18 shows an isometric view of an embodiment of a three-dimensionalmemory array of the present invention. FIGS. 1 through 18 arethree-dimensional isometric views illustrating the step-by-stepfabrication of the three-dimensional memory array shown in FIG. 18.FIGS. 19A, 19B, 19C and 19D show cross-sectional views of the memoryarray shown in FIG. 18 through the cross-sections XA-XA, XB-XB, YA-YAand YB-YB, respectively. The cross-section through diode strip DSU1 isthe same as the cross-section YA-YA and the cross-section through diodestrip DSL1 is the same as the cross-section YB-YB. Likewise, thecross-section through lower memory strip MSL2 is the same as thecross-section XA-XA and the cross-section through upper memory stripMSU2 is the same as the cross-section XB-XB.

FIG. 20 shows that same three-dimensional memory array from FIG. 18except that the dielectric layers have been removed so that thecomponents of the memory array can more clearly be seen. FIGS. 21A, 21B,21C and 21D show cross-sectional views of the memory array shown in FIG.20 through the cross-sections XA-XA, XB-XB, XC-XC and XD-XD,respectively. FIG. 23A shows a top view of the three-dimensional memoryarray shown in FIG. 20.

The step-by-step fabrication of the memory array shown in FIG. 18 andFIG. 20 will now be discussed. At each step of the process, the readeris also referred to the cross-sectional views XA-XA, XB-XB, YA-YA,YB-YB, shown in FIGS. 19A through 19D, respectively. Referring to FIG.1, a conductive layer 110 is formed on a semiconductor substrate 10. Thesubstrate 10 may be a conventional silicon monocrystalline substratewith a dielectric layer, such as silicon dioxide, deposited thereon.Alternately, the substrate 10 may be a silicon-on-sapphire substrate, adielectrically isolated substrate or a silicon-on-insulator substrate,each with a dielectric layer, such as silicon dioxide deposited thereon.The substrate 10 may include peripheral circuitry such as drivercircuitry and/or address circuitry.

An n+ type silicon layer 112 is then deposited on the conductive layer110 and an n type silicon layer 114 is deposited on the n+ type layer112. The layer 112 and the layer 114 are preferably deposited either asamorphous silicon or as polysilicon. Referring to FIG. 2, the n typelayer 114 may then be appropriately masked and portions of the n typelayer 114 may then be doped (using, for example, ion implantation ordiffusion techniques) to form p+ type strips 116 in the n type layer114. The p+ type strips 116 may have a width in the X-direction of about(5/3)F. “F” is the minimum photolithographic feature size. The minimumfeature size may be the limit achievable by lithographic techniques. Inone embodiment, the feature size F may be about 1000 Angstroms or less.

After the structure shown in FIG. 2 is completed so that the p+ strips116 are formed, the structure is subjected to a recrystallizationprocess. This process converts the amorphous silicon material orpolysilicon material of the n+ type layers 112, the n type layers 114and the p+ type layers 116 to a substantially monocrystalline material.

Referring to FIG. 3, the layers 110, 112, 114 and 116 are masked andetched to form lower diode strips DSL1 and DSL2. The diode strip DSL1includes a lower conductive line 110 a. The diode strip DSL1 alsoincludes a lower diode DL11 and a lower diode DL12 defined by thesemiconductor junctions between the n type layer 114 and the p+ typeregions 116. Diodes DL11 and DL12 are electrically coupled to theconductive line 110 a.

Likewise, diode strip DSL2 includes a lower conductive line 110 b. Thediode strip DSL2 also includes a lower diode DL21 and a lower diode DL22defined by the semiconductor junctions between the n type layer 114 andthe p+ type regions 116 of diode strip DSL2. Diodes DL21 and DL22 areelectrically coupled to the conductive line 110 b.

The conductive lines 110 a and 110 b may serve as address lines for thememory array. In the embodiment shown, the conductive line 110 a isdesignated as a first lower row line RL1 for the memory array while theconductive line 110 b is designated as a second lower row line RL2 forthe memory array. The lower row lines may also be referred to as lowerword lines. The lower row lines RL1 and RL2 are laterally spaced apartin the Y-direction. The space between the lines may be equal to thewidth of the row lines.

In an embodiment of the invention, the lower diodes strips DSL1 and DSL2may each be formed so as to have a width in the Y-direction of about(5/3)F. Hence, the corresponding lower row lines RL1, RL2 may have thesame width in the Y-direction of about (5/3F). The lower diodes DL11,DL12, DL21, DL22 may thus have a lateral dimension in the X-directionand a lateral dimension in the Y-direction of about (5/3)F.

As shown in FIG. 4, a dielectric material 150 is disposed between aswell as over the lower diode strips and planarized using chemicallymechanically polished (CMP) to form the structure shown in FIG. 4. Asshown in FIG. 4, the dielectric layer 150 includes a first portion 150 awhich fills the gaps between the diode strips DSL1 and DSL2 as well as asecond portion 150 b which is disposed above the diode strips. As shownin FIGS. 19A-D, the height of the second portion 150 b is represented asdZ1. The distance dZ1 represents the thickness of the second portion 150b. In another embodiment, a spin-on-glass (SOG) may be used to fill thevoids between the adjacent diode strips. In this case, alternativeplanarization approaches can be used, such as plasma etching, forexample. Other fill and planarization methods may be used.

Referring to FIG. 5, a conductive layer 210 is formed over thedielectric layer 150, an n+ type silicon layer 212 is formed over theconductive layer 210, and an n type silicon layer 214 is formed over then+ type layer 212. The n+ type layer as well as the n type layer may bedeposited as amorphous silicon or as polysilicon. The n type layer 214is then appropriately masked and doped to form p+ strips 216 as shown inFIG. 6 using patterning and dopant introduction techniques well known inthe art (where dopant introduction techniques include, for example, ionimplantation or diffusion techniques). The p+ strips 216 may have awidth in the X-dimension of about (5/3)F.

After the structure shown in FIG. 6 is completed so that the p+ strips216 are formed, the structure is subjected to a recrystallizationprocess. This process converts the amorphous silicon material orpolysilicon material of the n+ type layers 212, the n type layers 214and the p+ type layers 216 to a substantially monocrystalline material.

The conductive layer 210, n+ type layer 212, n type layer 214 and the p+strips 216 are then masked and etched to form the upper diode stripsDSU1 and DSU2 as shown in FIG. 7. The upper diode strip DSU1 includes aconductive line 210 a. The diode strip DSU1 also includes a first upperdiode DU11 and a second upper diode DU12 defined by the semiconductorjunctions between the n type layer 214 and the p+ type regions 216 ofupper diode strip DSU1. Diodes DU11 and DU12 are electrically coupled tothe conductive line 210 a.

Likewise, upper diode strip DSU2 includes a conductive line 210 b. Thediode strip DSU2 also includes a first upper diode DU21 and a secondupper diode DU22 defined by the semiconductor junctions between the ntype layer 214 and the p+ type regions 216 of diode strip DSU2. DiodesDU21 and DU22 are electrically coupled to the conductive line 210 b. Theconductive lines 210 a and 210 b may also serve as address lines for thememory array. In the embodiment shown, the conductive line 210 a isdesignated as a first upper row line RU1 for the memory array. Likewise,the conductive line 210 b is designated as a second upper row line RU2for the memory array. The upper and lower row lines RU1, RU2, RL1 andRL2 are laterally spaced apart in the Y-direction. The space between thelines may be equal to the width of the row lines. Hence, the spacebetween the lines may have a lateral distance in the Y direction whichmay be (5/3)F.

It is noted that the upper row lines RU1, RU2 are disposed above thelower row lines. In addition, the upper row lines RU1, RU2 are staggeredwith respect to the lower row lines RL1, RL2. The placement of the lowerand upper row lines alternate in the Y-direction so that a lower rowline is following by an upper row line and an upper row line is followedby a lower row line. In one embodiment, the upper and lower row linesmay not overlap at all. In another embodiment, there may be some overlapbetween the upper and lower row lines.

In an embodiment of the invention, the upper diode strips DSU1 and DSU2may be formed so as to have a width in the Y-direction of about (5/3)F.Hence, the corresponding row lines RU1, RU2 may also have a width in theY-direction of about (5/3)F. The diodes DU11, DU12, DU21, DU22 may thushave a lateral dimension in the Y-dimension equal to about (5/3)F aswell as a lateral dimension in the X-dimension of about (5/3)F.

It is noted that the upper diode strips DSU1, DSU2 are formed above thelower diode strips DSL1, DSL2. The upper and lower strips are separatedvertically by a distance dZ1 which is the thickness of the dielectriclayer portion 150 b. The upper diode strips DSU1, DSU2 are disposedabove the lower diode strips even through they do not overlap the lowerdiode strips in the embodiment shown in FIG. 7. In another embodiment ofthe invention, it is possible that the upper diode strips may overlapthe lower diode strips.

A dielectric material 250 is disposed between the upper diode stripsDSU1 and DSU2 as well as over the diode strips DSU1 and DSU2 and thenchemically mechanically polished (CMP) to form the planarized structureshown in FIG. 8. As shown, the dielectric layer 250 includes a firstportion 250 a disposed between the upper diode strips DSU1 and DSU2 aswell as a second portion 250 b disposed above the upper diode stripsDSU1 and DSU2. The height of the second dielectric portion 250 b isdesignated as dZ2 as shown in FIGS. 19A-D.

Referring to FIG. 9 (as well as the cross-sectional views shown in FIGS.19A and 19D), openings 264 are formed through the dielectric layer 250(which includes dielectric layer portions 250 a and 250 b) anddielectric layer portion 150 b so as to expose the top surfaces ofp+regions 116 of lower diodes DL11, DL12, DL21 and DL22. In theembodiment shown, the openings 264 have a circular cross section.However, in other embodiments, the openings may be formed to have asquare or rectangular cross section. The openings 264 may be formedusing standard lithographic techniques. The width (e.g. diameter) of theopenings 264 may be about one feature size F. In other embodiments ofthe openings may be made to have a smaller width (e.g. diameter) byplacing dielectric spacers along the sidewalls of the openings.

Referring to FIG. 10, each of the openings 264 are filled with aconductive material 266 (which is chemically mechanically polished) toform lower conductive plugs PL11, PL12, PL21 and PL22 that areelectrically coupled to the p+ material 116 of the lower diodes DL11,DL12, DL21 and DL22, respectively. The lower conductive plugs have awidth (e.g. diameter) that corresponds to the width of the openings 264.Hence, in an embodiment of the invention, the width (e.g. diameter) ofthe conductive plugs may be about one feature size F.

Referring to FIG. 11, a phase-change material 310 is then formed overthe top surface of the conductive plugs as well as over the top surfaceof dielectric layer 250 (more specifically, over dielectric layerportion 250 a). A conductive layer 312 is formed over the phase-changematerial layer 310.

Referring to FIG. 12, the layers 310 and 312 are masked and etched toform lower memory strips MSL1 and MSL2. Lower memory strip MSL1 ispositioned so that the phase-change material 310 of lower memory stripMSL1 is formed over the top surface of the lower conductive plug PL11and over the top surface of lower conductive plug PL21. Likewise, lowermemory strip MSL2 is positioned so that the phase-change material 310 oflower memory strip MSL2 is formed over the top surface of lowerconductive plug PL12 and over the top surface of lower conductive plugPL22. The lower memory strip MSL1 includes a conductive layer 312 awhich forms a first lower column line CL1 for the memory array.Likewise, the lower memory strip MSL2 includes a conductive layer 312 bwhich forms a second lower column line CL2 for the memory array.

In an embodiment of the invention, the lower memory strips MSL1 and MSL2may each be formed so as to have a width in the X-direction of about(5/3)F. Hence, the corresponding lower column lines CL1, CL2 may alsohave a width in the X-direction of about (5/3)F.

Referring to FIG. 13, a dielectric material 350 is deposited between thelower memory strips MSL1 and MSL2 as well as over the lower memorystrips MSL1, MSL2. The dielectric material is then chemicallymechanically polished. The dielectric layer 350 may be viewed as havinga first portion 350 a which fills the gaps between the memory strips aswell as a second portion 350 b which is formed above the memory strips.The height of the second portion 350 b is designated as dZ3 as shown inFIGS. 19A-D.

Referring to FIG. 14 (as well as cross-section shown in FIGS. 19B and19D), openings 364 are formed through the dielectric layer 350(including both dielectric layer portions 350 b and 350 a) as well asdielectric layer portion 250 b so that the openings 364 expose the topsurfaces of the p+ regions 216 of upper diodes DU11, DU12, DU21 andDU22. In the embodiment shown, the openings 364 have a circularcross-section. More generally, the cross-section may be of any shapeincluding, but not limited to, square and rectangular. In an embodiment,the openings 364 may have a width (e.g. a diameter) of about one featuresize F. In another embodiment of the invention, the openings may be madesmaller by forming dielectric sidewall spacers on the sidewalls of theopenings.

Referring to FIG. 15, each of the openings 364 are filled with aconductive material 366 and the conductive material is chemicallymechanically polished (CMP) to form upper conductive plugs PU11, PU12,PU21 and PU22 that are electrically coupled to the top surfaces of thep+ regions 216 of upper diodes DU11, DU12, DU21 and DU22, respectfully.The width of the conductive plugs corresponds to the width of theopenings 364. Hence, the width of each of the conductive plugs may beabout one feature size F.

Referring to FIG. 16, a phase-change material 410 is then formed overthe top surface of the upper conductive plugs as well as over dielectriclayer portion 350 b of the dielectric layer 350. A conductive layer 412is formed over the phase-change material layer 410.

Referring to FIG. 17, the layers 310 and 312 are masked and etched toform upper memory strips MSU1 and MSU2. The upper memory strip MSU1 ispositioned so that the phase-change material 410 of upper memory stripMSU1 is formed over the top surface of the upper conductive plug PU11and over the top surface of upper conductive plug PU21. Likewise, uppermemory strip MSU2 is positioned so that the phase-change material 410 ofupper memory strip MSU2 is formed over the top surface of upperconductive plug PU12 and over the top surface of upper conductive plugPU22. The upper memory strip MSU1 includes a conductive layer 412 awhich serves as a first upper column line CU1 for the memory array. Theupper memory strip MSU2 includes a conductive layer 412 b which servesas a second upper column line CU2 for the memory array.

In an embodiment of the invention, the upper memory strips MSU1 and MSU2may be formed so as to have a width in the X-direction of about (5/3)F.Hence, the corresponding upper column lines CU1, CU2 may also have awidth in the X-direction of about (5/3)F.

In the embodiment shown in FIG. 17, the upper memory strips MSU1, MSU2are disposed above the lower memory strips MSL1, MSL2. The upper memorystrips are vertically separated from the lower memory strips by thethickness of the dielectric layer portion 350 b which is dZ3. In theembodiment shown in FIG. 17, the upper memory strips are disposed abovethe lower memory strips even though the upper memory strips do notoverlap the lower memory strips. However, in alternate embodiments ofthe invention, it is possible that the upper and lower memory strips mayoverlap.

Referring to FIG. 18, a dielectric layer 500 is formed between uppermemory strips MSU1, MSU2 as well as over upper memory strips MSU1 andMSU2. The dielectric layer 500 may be viewed as having a first portion500 a disposed between the upper memory strips MSU1, MSU2 as well as asecond portion 500 b disposed above the upper memory strips MSU1, MSU2.

As noted above, FIG. 20 shows a view of the three-dimensional memoryarray from FIG. 18 except that all of the dielectric layers have beenremoved for clarity. FIG. 20 shows the lower diode strips DSL1, DSL2 aswell as the upper diode strips DSU1, DSU2. FIG. 20 also shows the lowermemory strips MSL1, MSL2 as well as the upper memory strips MSU1, MSU2.FIG. 20 also shows the lower plugs PL11, PL12, PL21, PL22 as well asupper plugs PU11, PU12, PU21, PU22.

FIGS. 21A, 21B, 21C, 21D show cross-sectional views of the memorystructure shown in FIG. 20 through the cross-sections XA-XA, XB-XB,XC-XC and XD-XD, respectively. The dashed lines are not part of thecross-section but show background components. For example, FIG. 21Ashows the memory structure in FIG. 20 through cross-section XA-XA whichgoes through the conductive plugs PL11 and PL21 (while plugs PU1, PU21and memory strip MSU1 are shown in the background). FIG. 21B shows thememory structure in FIG. 20 through the cross-section XB-XB which goesthrough the plugs PU11, PU21 (while plugs PL12, PL22 and memory stripMSL2 are shown as background). FIG. 21C shows the memory structure ofFIG. 20 through the cross-section XC-XC which goes through the plugsPL12, PL22 (while plugs PU12, PU22 are shown as background). Likewise,FIG. 21D shows the memory structure of FIG. 20 through the cross-sectionXD-XD which goes through the plugs PU12, PU22.

The lower memory strips MSL1 and MSL2 in combination with the four lowerconductive plugs PL11, PL12, PL21 and PL22 form four lower memoryelements ML11, ML12, ML21 and ML22, respectively. Referring to FIG. 21A,portions of lower column line CL1 and lower conductive plug PL11 serveas a top and bottom electrical contacts (also referred to aselectrodes), respectively, for the memory element ML11. Likewise,portions of lower column line CL1 and lower conductive plug PL21 serveas a top and bottom electrical contacts, respectively, for the lowermemory element ML21. Referring to FIG. 21C, portions of lower columnline CL2 and lower conductive plug PL12 serve as a top and bottomelectrical contacts, respectively, for the lower memory element ML12.Portions of lower column line CL2 and lower conductive plug PL22 serveas a top and bottom electrical contacts, respectively, for the memoryelement ML22. Lower memory elements ML11, ML12, ML21 and ML22 arephase-change memory elements comprising phase-change material 310.

The upper memory strips MSU1 and MSU2 in combination with the four upperconductive plugs PU11, PU12, PU21 and PU22 form four upper memoryelements MU11, MU12, MU21 and MU22, respectively. Referring to FIG. 21B,portions of upper column line CU1 and upper conductive plug PU11 serveas a top and bottom electrodes, respectively, for the memory elementMU1. Electrodes may also be referred to as electrical contacts. Portionsof upper column line CU1 and upper conductive plug PU21 serve as a topand bottom electrodes, respectively, for the upper memory element MU21.Referring to FIG. 21D, portions of upper column line CU2 and upperconductive plug PU12 serve as a top and bottom electrodes, respectively,for the upper memory element MU12. Portions of upper column line CU2 andupper conductive plug PU22 serve as a top and bottom electrodes,respectively, for the memory element MU22. Upper memory elements MUW1,MU12, MU21 and MU22 are phase-change memory elements comprisingphase-change material 410.

FIG. 22 is a schematic diagram of the three-dimensional memory arrayfrom FIGS. 18 and 20. Referring to FIG. 22, it is seen that each of thediodes is electrically coupled in series with a corresponding memoryelement to form a corresponding memory cell. Each of the lower diodes iselectrically coupled to a corresponding lower memory element. Diode DL11is electrically coupled in series with memory element ML11 by conductiveplug PL11 between row line RL1 and column line CL1. Diode DL12 iselectrically coupled in series with memory element ML12 by conductiveplug PL12 between row line RL1 and column line CL2. Diode DL21 iselectrically coupled in series with memory element ML21 by conductiveplug PL21 between row line RL2 and column line CL1. Diode DL22 iselectrically coupled in series with memory element ML22 by conductiveplug PL22 between row line RL2 and column line CL2.

Likewise, the upper diodes are electrically coupled to correspondingupper memory elements. Diode DU11 is electrically coupled in series withmemory element MU11 by conductive plug PL11 between row line RU1 andcolumn line CU1. Diode DU12 is electrically coupled in series withmemory element MU12 by conductive plug PU12 between row line RU1 andcolumn line CU2. Diode DU21 is electrically coupled in series withmemory element MU21 by conductive plug PU21 between row line RU2 andcolumn line CU1. Diode DU22 is electrically coupled in series withmemory element MU22 by conductive plug PU22 between row line RU2 andcolumn line CU2.

FIG. 22 shows that the three-dimensional memory array comprises fourdevice levels. The first device level Device Level 1 includes the fourlower diodes DL11, DL12, DL21 and DL22. The second device level DeviceLevel 2 includes the four upper diodes DU11, DU12, DU21 and DU22. Thethird device level Device Level 3 includes the four lower memoryelements ML11, ML12, ML21 and ML22. The fourth device level Device Level4 includes the four upper memory elements MU11, MU12, Mu21 and MU22.Each of the device levels is formed above the preceding device level.That is, Device Level 2 is formed above Device Level 1, Device Level 3is formed above Device Level 2, and Device Level 4 is formed aboveDevice Level 3. In one embodiment of the invention, each device level ofelements is arranged in a horizontally disposed layer above thesubstrate. Preferably, there is some vertical distance separating eachdevice level from the adjacent device level.

Referring again to the cross-sectional views of FIG. 19A through 19D(and, more specifically to FIGS. 19A and 19B), it is seen that the upperdiode strips DSU1, DSU2 are disposed above the lower diode strips DSL1,DSL2 and separated from the lower diodes strips DSL1, DSL2 by a distancedZ1 in the Z-direction. The distance dZ1 is equal to the thickness ofthe dielectric layer portion 150 b.

It is preferable that the thickness of the dielectric layer portion 150b is greater than 0 (so that the distance dZ1 is also greater than 0).It is possible that the thickness of the dielectric layer portion 150 bbe small or relatively thin.

FIGS. 19A-D also show that, it is seen that the lower memory stripsMSL1, MSL2 are disposed above the upper diode strips DSU1, DSU2 andseparated from the upper diode strips by a distance dZ2 in theZ-direction. The distance is equal to the thickness of the dielectriclayer portion 250 b. The distance dZ2 is preferably greater than 0 sothat the dielectric layer portion 250 b has some thickness, however, itis possible that dZ2 be relatively small such that the thickness of theof the dielectric layer portion 250 b is relatively thin.

It is also seen that the upper memory strips MSU1, MSU2 are disposedabove the lower memory strips MSU1, MSL2 and separated from the lowermemory strips by a distance dZ3 in the Z-direction. The distance dZ3 isequal to the thickness of the dielectric layer portion 350 b. Thedistance dZ3 is preferably greater than 0 so that the dielectric layerportion 350 b has some thickness. However, it is possible that dZ3 berelatively small such that the thickness of the dielectric layer portion350 b is relatively thin.

FIG. 23A shows a top view of the memory array structure from FIG. 20.FIG. 23A shows the lower diode strips DSL1, DSL2 (and the correspondinglower row lines RL1, RL2), the upper diode strips DSU1, DSU2 (and thecorresponding upper row lines RU1, RU2), the lower memory strips MSL1,MSL2 (and the corresponding lower column lines CL1, CL2), the uppermemory strips MSU1, MSU2 (and the corresponding upper column lines CU1,CU2), lower plugs PL11, PL12, PL21, PL22 and upper plugs PU11, PU12,PU21, PU22. FIG. 23A also shows the locations of the lower diodes DL11,DL12, DL21 and DL22 as well as the locations of the upper diodes DU11,DU12, DU21 and DU22.

Referring to FIG. 23A as well as FIGS. 19A-D it is seen that the upperrow lines RU1, RU2 are separated from the lower row lines RL1, RL2 inthe Y-direction by the distance dY1. Likewise, the upper diode stripsDSU1, DSU2 are laterally separated from the lower diodes strips DSL1,DSL2 in the Y-direction by a distance dY1.

Also, the upper column lines CU1, CU2 are separated from the lowercolumn lines CL1, CL2 in the X-direction by the distance dX1. Likewise,the upper memory strips MSU1, MSU2 are separated from the lower memorystrips MSL1, MSL2 in the X-direction by a distance dX1. In addition,each upper diode DU11, DU21, DU12, DU22 is separated from its closestneighboring lower diodes DL11, DL21, DL12, DL22 by a distance dX1 in theX-direction and a distance dY1 in the Y-direction.

In the embodiment shown in FIGS. 23A and 19A-D, dY1 is greater than 0.In this case, there is separation (and no overlap) between the upper rowlines RU1, RU2 and lower row lines RL1, RL2. Likewise, there isseparation (and no overlap) between the upper diode strips and the lowerdiode strips. In the embodiment of FIGS. 23A and 19A-D, dX1 is alsogreater than 0. In this case, there is separation (and no overlap)between the upper column lines CU1, CU2 and the lower column lines CL2,CL2. Likewise, there is also separation (and no overlap) between theupper memory strips DSU1, DSU2 and lower memory strips DSL1, DSL2.

However, in other embodiments of the invention it is possible that thedistances dX1 and/or dY1 be equal to 0. Also, in still furtherembodiments of the invention, it is possible that dX1 and/or dY1 be lessthan 0. If dY1 is less than 0, then the upper row lines overlap thelower row lines (and the corresponding upper diode strips overlap thelower diode strips). If dX1 is less than 0, then the upper column linesoverlap the lower column lines (and the upper memory strips overlap thelower memory strips).

FIG. 23B shows an embodiment of the invention where dY1 is less than 0and dX1 is greater than 0. In this embodiment, the upper row linesoverlap the lower tow lines (and the corresponding upper diode stripsoverlap the corresponding lower diode strips). Another embodiment of theinvention is shown in FIG. 23C where dY1 is greater than 0 and dX1 isless than 0. In this case, the upper column lines overlap the lowercolumn lines (and the corresponding upper memory strips overlap thecorresponding lower memory strips).

Another embodiment of the invention is shown in FIG. 23D where both dX1is less than 0 and dY1 is less than 0. In this case, the upper row linesoverlap the lower row lines and the upper column lines overlap the lowercolumn lines. Likewise, the upper diode strips overlap the lower diodestrips and the upper memory strips overlap the lower memory strips. Inaddition, when both dX1 is less than 0 and dY1 is less than 0, then theupper diodes overlap the lower diodes.

FIGS. 19A′ and 19B′ shows cross-sections of the memory array through thecross-sections XA-XA and XB-XB, respectively when dY1 is less than 0 andthe upper and lower row lines overlap (and the corresponding upper andlower diode strips overlap). Referring to FIG. 19A′, it is seen howupper diode strip DSU1 overlaps lower diode strip DSL1 and also overlapslower diode strip DSL2. It is also seen how upper diode strip DSU2overlaps lower diode strip DSL2. Likewise, FIGS. 19C′ and 19D′ showcross-sectional views of the memory array through the cross-sectionYA-YA, YB-YB, respectively when dX1 is less than 0 and the upper andlower column lines overlap (and the corresponding upper and lower memorystrips-overlap).

It is noted that the upper and lower row lines may overlap provided thatthe upper row lines do not contact the conductive plug material 266.Likewise, the upper and lower column lines may overlap provided that thelower column lines do not contact the conductive plug material 366.Overlap of the row lines and/or the column lines may be used to furtherincrease the density of the memory array.

In all of the embodiments shown in the top view of FIGS. 23A-D (as wellas cross-sectional views of FIGS. 19A-D and the cross-sectional views ofFIGS. 19A′-D′) the upper row lines and lower row lines are staggered inthe Y-direction. The upper and lower row lines alternate such that alower row line (e.g. RL1) is followed by an upper row line (e.g. RU1),and an upper row line (e.g. RU1) is following by a lower row line (e.g.RL2).

Also, the upper column lines and lower column lines are staggered in theX-direction. The upper and lower column lines alternate such that alower column line (e.g. CL1) is followed by an upper column line (e.g.CU1), and an upper column line (e.g. CU1) is following by a lower columnline (e.g. CL2). Hence, a staggered arrangement of the row lines may beachieved with or without overlap. Likewise, a staggered arrangement ofthe column lines may be achieved with or without overlap.

In the embodiments shown in FIG. 23A-D, the footprints (e.g. projectionsonto the substrate) of the lower diodes form a checkerboardconfiguration with the footprints of the upper diodes. This checkerboardconfiguration may be achieved with or without overlap between the upperand lower diodes. In the embodiments shown in FIGS. 23A-D, the lowerdiodes DL11, DL12, DL21, DL22 are arranged in rows and columns.Likewise, the upper diodes DU11, DU12, DU21, DU22 are also arranged inrows and columns. The upper diodes are staggered with respect to thelower diodes. Also, the upper diodes are staggered diagonally withrespect to the lower diodes. The upper diodes alternate with the lowerdiodes along diagonals. For example, the lower diode DL11 is followed bythe upper diode DU11. The upper diode DU11 is followed by a lower diodeDL22. The lower diode DL22 is followed by the upper diode DU22.

Likewise, in the embodiments shown in FIGS. 23A-D, the upper memoryelements may be staggered with respect to the lower memory elements.Likewise, in the embodiments shown in FIGS. 23A-D, the upper memoryelements may alternate with the lower memory elements along thediagonals. The lower memory elements may be diagonally staggered withrespect to the upper memory elements.

In the embodiment of the invention shown in FIGS. 23A-D, the lower rowlines RL1, RL2 and the upper row lines RU1, RU2 each have a width W_(y)in the Y-direction (likewise, the corresponding lower diode strips andthe upper diode strips each have a width W_(y) in the Y-direction. Also,the lower column lines CL1, CL2 and the upper column lines CU1, CU2 eachhave a width W_(x) the X-direction (likewise, the corresponding lowermemory strips and upper memory strips each have a width W_(x) in theX-direction). The lower and upper diodes have a lateral dimension W_(x)in the X-direction and a lateral dimension W_(y) in the Y-direction. Thedimension W_(x) as well as the dimension W_(y) may be the same dimensionW. In an embodiment of the invention, the dimension W may be around(5/3)F. It is noted that in general, the dimensions (e.g., widths,lengths, heights, thicknesses) of each of the row lines, column lines,diode strips, memory strips, diodes and memory elements is not limitedto any particular dimension. In an embodiment of the invention, it iseven possible that the widths of two or more column lines (or two ormore row lines) are different.

FIG. 25A shows a top view of the three-dimensional memory array fromFIG. 23A. FIG. 25A shows a top view of a unit volume 1010 of thethree-dimensional array. The unit volume includes two memory cells. Theunit volume 1010 has a lateral dimension in the X-direction of2W_(x)+2dX1. The unit volume 1010 has a lateral dimension in theY-direction of 2W_(y)+2dY1. Since the unit volume includes two memorycells, the cell size is one-half the size of the footprint of the unitvolume.dX1>0 and dY1>0  Case 1(separation in both the X-direction and Y-direction)

In this case,

footprint size is 4(W_(x)+dX1)(W_(y)+dY1), and

cell size is 2(W_(x)+dX1)(W_(y)+dY1)

It is seen that even though dX1>0 and dY1>0, the cell size of thethree-dimensional array may be made less than 4(W_(x)) (W_(y)) byappropriately choosing the values of dX1 and dY1 (e.g. making them smallenough). Likewise, the cell size of the three-dimensional array may bemade less than 3(W_(x)) (W_(y)) by choosing the appropriate values ofdX1 and dY1.dX1=dY1=0  Case 2In this case,

footprint size is 4(W_(x))(W_(y)), and

cell size is 2(W_(x))(W_(y))dX1<0 and/or dY1<0  Case 3(overlap in either the X-direction and/or Y-direction)In this case,

footprint size is less than 4(W_(x))(W_(y)), and

cell size is less than 2(W_(x))(W_(y))

An example of a two-dimensional array is shown in FIG. 25B. In thisexample, there are only two row lines R1, R2 and only two column linesC1, C2. Each of the row lines may also have a width W_(y) in theY-dimension and each of the column lines may also have a width W_(x) inthe X-dimension. There is also a space of width W_(x) between each ofthe column lines and a space of width W_(y) between each of the rowlines. FIG. 25B shows the unit volume 1020 of the two-dimensional array.The unit volume includes a single memory cell and has a lateraldimension in the X-direction of 2W_(x) and a lateral dimension in theY-direction of 2W_(y). The total footprint area of the unit volume 1020is 4(W_(x))(W_(y)). Since, the unit volume 1020 includes only a singlememory cell, the cell size of the array is also 4(W_(x))(W_(y)). In anembodiment of the invention, W_(x)=W_(y)=W which may be about (5/3)F.

Hence, it is seen that the three-dimensional memory array of the presentinvention may have memory cell size which is less than the memory cellsize of a two-dimensional array. This is an advantage of thethree-dimensional memory array.

Another advantage of the three-dimensional memory array is that sincethe memory cells need not contact the substrate, the substrate isavailable for use other than for defining the memory cells. In oneembodiment of the present invention, the area in the substrate may beused for at least portions of the row decoders, column decoders, I/Omultiplexors, and read/write circuits. This helps to minimize thefraction of the die surface area not devoted to memory cells.

In the embodiments of the memory array shown in FIGS. 23A-D, there aretwo lower row lines RL1, RL2 and two upper row lines RU1, RU2. In otherembodiments of the invention, the memory array may include more than twolower row lines and more than two upper row lines. More generally, thememory array of the present invention may include at least one lower rowline and at least one upper row line.

Likewise, in the embodiments of the memory array shown in FIGS. 23A-D,there are two lower column lines CL1, CL2 and two upper row lines CU1,CU2. In other embodiments of the invention, the memory array may includemore than two lower column lines and more than two upper column lines.More generally, the memory array of the present invention may include atleast one lower column line and at least one upper column line.

In an embodiment of the invention, a lower memory cell may beelectrically coupled between each of the lower row lines and each of thelower column lines. Likewise, an upper memory cell may be electricallycoupled between each of the upper row lines and each of the upper columnlines. Each lower memory cell includes a lower phase-change memoryelement in series with a lower diode. Each upper memory cell includes anupper phase-change memory element in series with an upper diode. It ispossible that upper and lower diodes be replaced with other types ofisolation elements. In an embodiment, there may be at least one uppermemory cell. In an embodiment, there may be a plurality of upper memorycells. In an embodiment, there may be at least one lower memory cell. Inan embodiment, there may be a plurality of lower memory cells.

Referring again to FIGS. 3 and 7, it is noted that the conductive lines110 a, 110 b and 210 a, 210 b were designated as the row lines for thememory array while the conductive lines 312 a, 312 b and 412 a, 412 bwere all designated as the column lines for the memory array. In analternate embodiment of the invention, it is possible that theconductive lines 110 a, 110 b and 210 a, 210 b may be designated as thecolumns lines for the memory array while the conductive lines 312 a, 312b and 412 a, 412 b may be designated as the row lines for the array.

In addition, in the embodiments shown in FIG. 7, the conductive lines312 a, 312 b and 412 a, 412 b are perpendicular to the conductive lines110 a, 110 b and 210 a, 210 b. However, in other embodiments of theinvention, they may not be perpendicular. Instead they may merely crosseach other at some angle.

Referring to FIG. 2, in an alternate embodiment of the invention, layer112 may be formed of a p+ type material, layer 114 may be formed of a ptype material and layer 116 may be formed of an n+ type material.Likewise, referring to FIG. 6, layer 212 may be formed of a p+ typematerial, layer 214 may be formed of a p type material and layer 216 maybe formed of an n+ type material.

In one or more other embodiments of the invention, the lower diodes andupper diodes may be formed in other ways. In addition, the lower diodesand upper diodes may be replaced with other types of isolation devices.For example, other types of isolation devices include, withoutlimitation, transistors and threshold switches (such as chalcogenidethreshold switches and S-type threshold switches).

It is noted that one or more additional conductive layers may placedbetween the phase-change material and the conductive plugs to form othertypes and structure for the bottom electrodes of the memory elements.Likewise, one or more additional conductive layers may be placed betweenthe phase-change material and the column lines CL1, CL2, CU1 and CU2 toform other types and structure for the top electrodes of the memoryelements. FIG. 24 shows the use of a lower electrode 700 formed byplacing a conductive material within a smaller opening 660 defined bydielectric sidewall spacer 600. The material is planarized to form asmaller plug. The electrode 700 provides for a smaller area of contactwith the phase-change material than that provided by the plug PL11.

Examples of the materials which may be used for conductive layers 110,210, 312, 412 as well as for materials 266 and 366 include, but is notlimited to, titanium nitride, titanium aluminum nitride, titaniumcarbonitride, titanium silicon nitride, molydenum, molybdenum nitride,carbon, carbon nitride, tungsten, tungsten silicide, titanium-tungsten,n-type doped polysilicon, p-type doped polysilicon, n-type doped siliconcarbon compounds and/or alloys, p-type doped silicon carbon compoundsand/or alloy.

Examples of materials which may be used as the dielectric layers 150,250, 350 and 500 include oxides and nitrides. Examples of oxides includesilicon oxide. Examples of nitrides include silicon nitride.

The memory material may be a phase-change material. The phase-changematerials may be any phase-change memory material known in the art. Thephase-change materials may be capable of exhibiting a first order phasetransition. Examples of materials are described in U.S. Pat. Nos.5,166,758, 5,296,716, 5,414,271, 5,359,205, 5,341,328, 5,536,947,5,534,712, 5,687,112, and 5,825,046 the disclosures of which are allincorporated by reference herein.

The phase-change materials may be formed from a plurality of atomicelements. Preferably, the memory material includes at least onechalcogen element. Hence, the phase-change material may be achalcogenide material. The chalcogen element may be chosen from thegroup consisting of Te, Se, S and mixtures or alloys thereof. The memorymaterial may further include at least one element selected from thegroup consisting of Ge, Sb, Bi, Pb, Sn, As, S, Si, P, O, and mixtures oralloys thereof. In one embodiment, the memory material comprises theelements Te, Ge and Sb. In another embodiment, the memory materialconsists essentially of Ge, Sb and Te. An example of a memory materialwhich may be used is Ge₂Sb₂Te₅.

The memory material may include at least one transition metal element.The term “transition metal” as used herein includes elements 21 to 30,39 to 48, 57 and 72 to 80. Preferably, the one or more transition metalelements are selected from the group consisting of Cr, Fe, Ni, Nb, Pd,Pt, Co, Ti and mixtures or alloys thereof. The memory materials whichinclude transition metals may be elementally modified forms of thememory materials in the Te—Ge—Sb ternary system. This elementalmodification may be achieved by the incorporation of transition metalsinto the basic Te—Ge—Sb ternary system, with or without an additionalchalcogen element, such as Se.

It is to be understood that the disclosure set forth herein is presentedin the form of detailed embodiments described for the purpose of makinga full and complete disclosure of the present invention, and that suchdetails are not to be interpreted as limiting the true scope of thisinvention as set forth and defined in the appended claims.

1. A memory array comprising: a substrate; a first phase-change memorycell disposed over said substrate, said first phase-change memory cellincluding a first lower contact in series with a first phase-changelayer and a first upper contact in series with said first phase-changelayer, said first phase-change layer being interposed between said firstlower contact and said first upper contact; and a second phase-changememory cell disposed over said substrate, said second phase-changememory cell including a second lower contact in series with a secondphase-change layer and a second upper contact in series with said secondphase-change layer, said second phase-change layer being interposedbetween said second lower contact and said second upper contact; whereinsaid second lower contact is disposed between said first lower contactand said first upper contact.
 2. The memory array of claim 1, whereinsaid second lower contact is disposed above said first phase-changelayer.
 3. The memory array of claim 1, wherein said first phase-changelayer is disposed above said first lower contact and said secondphase-change layer is disposed above said second lower contact.
 4. Thememory array of claim 1, wherein said first phase-change memory cellfurther includes a first isolation device in series with said firstphase-change layer, said first isolation device being interposed betweensaid first lower contact and said first upper contact and wherein saidsecond phase-change memory cell further includes a second isolationdevice in series with said second phase-change layer, said secondisolation device being interposed between said second lower contact andsaid second upper contact.
 5. The memory array of claim 4, wherein saidfirst isolation device is a diode or transistor.
 6. The memory array ofclaim 4, wherein said first isolation device is a threshold switch. 7.The memory array of claim 4, wherein a first conductive pluginterconnects said first isolation device and said first phase-changelayer.
 8. The memory array of claim 7, wherein a second conductive pluginterconnects said second isolation device and said second phase-changelayer.
 9. The memory array of claim 8, wherein said first conductiveplug is electrically isolated from said second conductive plug.
 10. Thememory array of claim 4, wherein the projection of said second isolationdevice onto said substrate overlaps the projection of said firstisolation device onto said substrate.
 11. The memory array of claim 1,wherein said second lower contact is electrically isolated from saidfirst upper contact.
 12. The memory array of claim 1, wherein adielectric material is disposed between said second lower contact andsaid first upper contact.
 13. The memory array of claim 1, wherein saidsecond lower contact is disposed between said first lower contact andsaid first phase-change layer.
 14. The memory array of claim 1, whereinsaid second upper contact is disposed above said first upper contact.15. The memory array of claim 1, wherein the projection of said secondphase-change layer onto said substrate overlaps the projection of saidfirst phase-change layer onto said substrate.